Ellipsometry is a technique of non-destructive measurement allowing optical characterisation of a sample having a specular or quasi-specular surface.
Ellipsometry can be implemented in situ and allows then the study of the mechanisms of thin layer growth, the formation of interfaces and the control of the carrying out method of said layers and interfaces. Ellipsometry is, for example, used for the study and the control of the manufacture of semi-conductors.
The ellipsometric measurements can be made at one fixed wavelength or at several wavelengths (spectroscopic ellipsometry). According to the wavelength region of the source, near ultra violet, visible, near infra red, infra red, etc., it is possible to have access to different properties of the layers, of the materials or to explore different materials.
In the ultra violet and visible regions, the depth of penetration of the radiation is often slight. This constitutes favourable conditions for the study of surfaces and interfaces, and for real time controls. Generally this does not allow access to layer volumic properties and materials which can, on the other hand, be obtained through measurements in the infra red region.
Infra red is well suited to measurements of vibrational absorption (chemical bonds).
In order to carry out ellipsometric measurements, the surface of a sample is illuminated by a light beam and the state of polarisation of an incident beam i is compared with that of the reflected beam r or the transmitted beam. A polarisation vector E is generally represented by its projections E.sub.S and E.sub.P respectively perpendicular and parallel to the plane of incidence. The projections E.sub.S and E.sub.P are complex amplitudes.
In the domain of ellipsometry, the relationship p=(E.sub.P /E.sub.S).sup.r /(E.sub.P /E.sub.S).sup.i, indicative of the modifications of the state of polarisation produced by the surface being studied, is generally represented in the form: EQU p=tg.PSI..multidot.exp (i.DELTA.)=(E.sub.P /E.sub.S).sup.r /(E.sub.P /E.sub.S).sup.i
The two angles .PSI. and .DELTA. describing the change in polarisation are thus combined in the complex quantity p.
The angles .PSI. and .DELTA., and hence the number p, depend, at the same time, on the properties of the sample, the angle of incidence of a beam and the measurement wavelength. The expression of .PSI. and .DELTA., or of p, as a function of these parameters, is given by the equations of Fresnel quoted, for example, by D. CHARLOT and A. MARUANI in Appl. Opt. 24, 3368, 1985.
In a phase modulation ellipsometer, an incident beam has its polarisation modulated by a phase difference generated between two appropriate axes by a phase modulator. The phase shift .delta.(t) develops typically with time t in accordance with a periodic pulse law .omega., .delta.(t) being proportional to the first order to sin(.omega.t).
In a phase modulation ellipsometer, the intensity of a luminous flux reflected by a sample allows, in a known way, the values of .PSI. and .DELTA. to be deduced.
Ellipsometry, and more particularly Spectroscopic Phase Modulated Ellipsometry (SPME) is a high performance technique for the measurement, in real time, of the growth of layers on a substrate. This technique has the advantage of not disturbing reactions in progress. Furthermore, it is very sensitive to the physical parameters of the measured sample, such as a thickness d of layer and an index of refraction n. Furthermore it enables measurements to be made rapidly.
In accordance with a known method, the angles .PSI. and .DELTA. or p are deduced from intensity measurements. These quantities .PSI. and .DELTA. depend on the physical parameters of the measured sample such as the refractive index n and the thickness d of the upper layer. These parameters can therefore be calculated subsequently from .PSI. and .DELTA. by direct inversion of the Fresnel equations. This inversion must generally be carried out in an iterative manner.
The application of Spectroscopic Phase Modulated Ellipsometry, in situ, for diagnosis and for growth control, is, for example, described in the document "High Speed Spectral Ellipsometry for In Situ Diagnostics and Process Control.sub.--, DUNCAN et al., J. Vac. Sci. Technol. B., 12(4), 1984.
Despite its effectiveness, this method has the disadvantage, in certain circumstances, of generating uncertainties in the measurements of physical parameters. These uncertainties can arise, in particular, during growth of transparent material on an absorbent substrate. The accuracy of the measurements is thus substantially deteriorated.